I have 2 children (8 &10) from my 1st marriage and my fiance has an 18 yr old that he adopted when the boy was in 6th grade. We would really like to do some special during the ceremony to unite us as a family. Any ideas?

I've seen some really lovely moments where the children help light a unity candle or when you say 'i do', the kids also say 'i do'. It might even be nice to have your 2 children walk you down the aisle, one on either side. Perhaps the 18 yr old could participate by doing a reading or making a special toast at the reception, as well. Have you asked them what they would like to do? They might have their own special ideas!

My uncle married a woman with a 12y/o daughter. During the ceremony they had her come to the front and they gave her a necklace and the minister said a few words about joining the families. Found this site that looked like it had good suggestions:
http://www.idotaketwo.com/second_marriage_ceremony.html

Allow your children to come up and sign part of the official documents at the signing of the register. Yes you need two witnesses for the license but your officiant will give you 'the record of marriage' part of the license and anyone can sign, print or make their mark! Warn your wedding photographer to be on the alert for these wonderful moments. In my experience, children love this above all other part of the ceremony, and so does everyone else .Again when your officiant introduces you have your children up there with you. They will love the applause and treasure the memory of how important they were on Mom or Dad's big day.

If presenting roses are a part of the ceremony the couple will have a rose for each of the children. After exchanging roses, the couple will then give each child a rose, a hug, and whisper "I love you." Typically, couples may give children a gift right after they exchange their own rings and vows - usually a necklace, medallion, or ring - along with a hug and an "I love you."
http://www.tmclark.com/JP/children.html

something to think about... Word it wisely.
Be very careful about your vows so that you'll not encounter this. Both had previously been married. Ben had 2 kids ages 8 and 12 and Barbara a daughter age 5. Barbara presented her vows to the children in a way that required a response. This is what Barbara said: " I hope that you'll accept my promise to be the best mom I can be to you. I promise to love you and support you and be fully involved in your life." Barbara wanted to make her vows less solemn and more appealing to the kids, so she ended with "Do we have a deal?" What a mistake!!! The 8 years old did not answer but the 12 years old did. In front of all the guests he said: "You may love us & support us but you'll never be our mom. We already have a mom." Lesson learned! Vows should always be one sided. They are to be given with no strings attached and no expectations. Your vows are your promise. Don't put them in a position where they have to protect

We have 3 boys (14, 10, & 7 at the time of the wedding)... we are doing a sand photo frame in which each of the kids have picked the color of the sand, and we have ours... each year on our anniversary we are going to do a family pic (just wherever) and replace it... we are also have names engraved on their vases that will stay on the mantle with the frame... as the kids grow, the picture will reflect those who continue to reside in the house!!!!

I would never suggest asking the kids to take a vow of any sort. The sand is great, a handfasting with several cords (corresponding to the kids' personalities) would be fun too, but make sure they are absolutely comfortable with the choices.

I have seen the kids of a joining family get asked "Do you give your blessing?" by the officiant after the vows are said. (they respond I/We do)

then the officiant says: I now pronounce you a family

OR as an alternative:
When my mother and step-dad got married, my mom asked me to be her Maid of Honor (even though I was 6) I felt very included. I think their officiant said the I pronounce you a family bit also.

My FH has 2 girls (14 and 9) and I'm not really sure what I want to do for the ceremony. The younger one is gung-ho about being included and the older girl is more of a "stick-to-the-shadows" personality. I don't want to upset either of them by doing/not doing something. It is really important to me that I show them how much they mean to me, but i don't want either of them to be embarrassed.